In humans, normal (trichromatic) color vision is conferred by the presence of three populations of cone photoreceptor cells in the retina of the eye. The retina also contains rod photoreceptor cells that detect the brightness (i.e., luminosity) of incident light. Rods function primarily at night and under low light (i.e., scotopic) conditions; whereas cones function at the higher intensities typically present during daylight hours (i.e., photopic conditions). It is the cones, rather than rods, that are responsible for generating our sense of color. The cone cells contain photosensitive pigments, and in different populations of cone cells, the pigments are maximally sensitive to different wavelengths of light. The three human types of cone cell have absorption maxima at approximately 420 nm, 530 nm and 560 nm, and are described as blue-absorbing, green-absorbing and red-absorbing respectively, corresponding to the color of light at the absorption maxima. Because of their different absorption spectra, the three classes of pigments absorb light of any given wavelength to different extents. The differential absorption of the three classes of cells is transmitted to the brain, and the information processed from this signal generates human perception of color. If all three photopigments are stimulated about equally, as by incidental light containing a mix of all visual wavelengths, no differential signal reaches the brain, and the light appears colorless. Colorless light is seen as white or a shade of gray, depending on its intensity and the background illumination.
The color vision conferred by the three human cone populations is dependent upon those portions of the electromagnetic spectra that reach the retina. Before reaching the retina, light must pass through the cornea, lens and vitreous humor. In humans, the yellowish coloration of the lens acts as a "cut-off" filter, effectively limiting the transmission of short wavelength blue and near ultraviolet light to the retina. Thus, humans have very low sensitivity to light of these wavelengths.
The color vision of many nonhuman vertebrates differs from that of humans in several respects. Most notably, many mammals, including deer, pigs, cows, other ungulates, rabbits, squirrels, dogs and cats have only two populations of cone photoreceptors compared with three in humans. Pigs, for example, have two photopigments with absorption maxima at about 440 nm and 560 nm (Neitz et al. (1989), Visual Neuroscience 2: 97-100). These species are said to possess dichromatic vision. Dichromatic vision results in a very limited color perception compared with trichromatic. Whereas trichromatic humans can perceive several hundred color gradations from different wavelengths in the visible spectrum, dichromatic animals can perceive only two distinct colors with gradations of colorlessness in between. Thus, at low wavelengths of incident light, a dichromat perceives a blue color. As the wavelength is raised, the intensity of blue color decreases. Eventually, the blue color completely disappears and the light appears entirely colorless. On further increasing the wavelength, an increasing intensity of yellow appear, until eventually the yellow light appears relatively pure (i.e., saturated). The wavelength at which light appears entirely colorless, untinted by either blue or yellow coloration, is that at which the two populations of cone cells are equally stimulated. This wavelength is known as the neutral point. The colorless light, at or around the neutral point, is perceived as white or a shade of gray, depending on its intensity and the background illumination.
A further notable difference in vision between many nonhuman vertebrates and humans, is that the former lack the human's yellow coloration of the lens of the eye. In nonhuman vertebrates lacking the yellow coloration, short wavelength blue and ultraviolet light that would be filtered out in humans, reaches the nonhuman's retina. Thus, some nonhuman vertebrates have much greater sensitivity that humans to short wavelength light.
Traditional camouflages for human observation of animals have not exploited the differences in color vision of humans and animals. A traditional camouflage might comprise a mixture of browns and greens to simulate the forest background against which a human observer would be perceived by an animal. Such a camouflage may indeed make a human inconspicuous to animals. The difficulty with this approach is that a person so camouflaged is equally inconspicuous to other humans. When other humans are engaged in hunting, this presents a dangerous situation for the camouflaged human being of being mistaken for a target animal. Indeed, several fatal and crippling accidents have been reported. See, e.g. Gillins, UPI Report (Oct. 1, 1986).
The high incidence of hunting accidents from use of traditional camouflages has led the legislatures of many states to require hunters to wear clothing comprising "Hunter's Orange" (otherwise known as "daylight fluorescent orange" fabric). This fabric must emit at least 85% of luminance in a narrow band of wavelengths ranging between 595-605 nm and in addition, have at least a 40% luminosity factor. This band of wavelengths is near the peak of human visual sensitivity at 555 nm (Wysecki and Stiles (1982)). Thus, use of Hunter's Orange results in a fabric that is highly visible to humans and helps to avoid accidents. However, as revealed by the present disclosure, Hunter's Orange contrasts strongly with a dichromatic animal's perception of a natural background. Thus, Hunter's Orange fabrics achieve safety at some cost to utility and are far from ideal for assembly of camouflage clothing.
A product termed U-V-Killer.TM. solution (Atsko/Sno-Seal Inc., Orangeburg, S.C. 29115) has recently been reported for treating fabrics (blaze orange or otherwise) to reduce conspicuousness to animals. The problem sought to be addressed by treatment with the product is the reflection of ultraviolet irradiation caused by trace amounts of brighteners present in the fabric. Mandile, Outdoor Life (July, 1990) pp. 81-88. The traces of brighteners are absorbed by the fabric when it is washed in conventional detergent. U-V-Killer.TM. solution (Atsko/Sno-Seal Inc., Orangeburg, S.C. 29115) allegedly blocks the ultraviolet irradiation emitted by the brighteners. However, under daylight illumination the contribution of trace amounts of brighteners to total emissions is probably insignificant. Thus, treatment with U-V-Killer.TM. solution (Atsko/Sno-Seal Inc., Orangeburg, S.C. 29115), which does not change the residual spectrum of light emitted by conventional camouflage materials without brighteners, will not appreciably affect an animal's perception of these materials under daylight illumination.
Therefore, a need exists for a camouflage fabric that appears highly conspicuous to humans and yet blends into the background as perceived by dichromatic animals, particularly deer, under normal daylight illumination. The present invention exploits differences in color vision between trichromatic humans and deer to fulfill this and other needs.